appliance stores sell a special paint, probably mostly special so the
material doesnt outgass when heated and contaminate your food...
might be time to consider a new microwave, yours is getting elderly..
at least run your finger around the closed door while the ovens heating
, if you find your finger gets warm get oven serviced or replaced.
Old appliances can become a hazard, espically microwaves...
hinges wear and seals go bad after nearly 20 years.
BTW I tend to hang onto stuff forever, but old semi hazardous ones